1. Field of the Invention
The present invention relates to a melting temperature adjustable metal thermal interface material (TIM) and a packaged semiconductor including thereof.
2. Description of Related Art
Packaged semiconductors such as LEDs having high luminance, power insulation gate transistor, graphics chip, central processing units (CPUs) and so on generate higher and higher heat flow density during operation or operate at high temperature circumstance, resulting in the generation of potential hot spots. Thus, it is necessary to reduce the high heat flux so as to drop the operation temperature of the packaged semiconductors under the maximum junction temperature thereof. In order to prevent overheating of chips, current technology of packaged semiconductors have been developed toward one of either a multiple cores, dynamic voltage and/or frequency scaling, miniaturization in integrated circuits, or a combination of these varieties to reduce the generated heat energy during operation. In addition, a heat dissipation device with high heat radiation performance and a TIM with low interface thermal resistance (or labeled as “thermal impedance”) may also be applied to improve heat dissipation of the packaged semiconductors. The latter is a simpler and achievable solution.
The position of the TIM applied in the packaged semiconductors includes, for example, a first-stage thermal interface between an integrated circuit (IC) die and the heat spreader lid inside a packaged IC or a second-stage thermal interface between the heat spreader lid and a heat dissipation device outside the packaged IC. Thermal greases and low melting point alloys (LMAs) possess low interface thermal resistance among all kinds of TIMs currently. However, thermal greases are prone to dry out at severe high-temperature circumstance gradually, or they may be extruded out the thermal interface little by little due to a cycling thermal stress generated when the packaged semiconductors operate at varying power levels, finally resulting in a poor thermal contact resistance of thermal interfaces. The LMAs suitable for thermal interface have typically a property of phase change from solid to slurry in the interface or act like cream at solid state, and thus they conform to the interface surface throughout. Although LMAs could outperform thermal greases with better interfacial heat-conduction performance because of the benefit of melting/solidification reaction caused by interface temperature fluctuation, which promote large absorption and dispersion of joule heat passing through the interface. Conventional LMAs are either eutectic alloys, near eutectic alloys, or pure metals of easily deformed, for example, pure indium. Due to melting effects of eutectic composition or pure metal, consequently conventional LMAs have quite narrow temperature range of melting and are unable to adjust inherently melting points to best meet interface requirements of varying packaged semiconductors. According to the match issue for junction temperature range of the packaged semiconductors, most of the LMAs are not suitable for thermal interface application due to their fixed eutectic temperatures and narrow range of melting temperatures. After accomplishing the “Restriction of the use of certain hazardous substance in EEE (ROHS)” regulated by the European Union, the alloy consisting of Pb and/or Cd and parts or all of Bi, In, Sn and so on is excluded, and thus selection of the LMAs suitable for thermal interface application is more reductive.
The statement about the limitation of conventional LMAs for thermal interface application is as follows.
The first-stage thermal interface material between the IC die and the heat spreader lid connected thereof has to adequately accommodate the thermal stress or twist deformation caused by thermal expansion mismatch between the IC die and the heat dissipation device, besides jointing said two elements and maintaining a low interface thermal resistance. For conventional LMAs possessing a melting temperature higher than a junction temperature range of the IC die, said solid LMAs often have intolerable Young's modulus up to tens of Gpa and are hard to accommodate the thermal stress inside the package, and consequently the operating IC die may take the risk of early fatigue fracture. Because of above-mentioned problem, most of LMAs are excluded except for soft metals such as pure indium or In—Ag alloy composed of low solutes. If a conventional LMA having a melting temperature less than or within the junction temperature range of the IC die is used, the LMA will be easily heated to slurry state with high fluidity due to its narrow temperature range of melting; therefore it is difficult to joint the IC die and the heat dissipation device steadily. Although indium or In—Ag alloy composed of low solutes are currently utilized to the first-stage thermal interface, the soft metal is designed to keep solid state throughout during operation of the IC die, so it is hard to achieve the latent heat effect of melt for absorbing large heat energy. In addition, pure indium or In—Ag alloy may be overheating and prone to generate shrinkage voids within joint interface layer if the solder reflow process of chip package is performed utilizing a lead-free solder with melting temperature more than 220° C. In some cases, there exist a need to utilize a metal TIM having quite a wide temperature range of melting and a liquidus temperature greater than that of pure indium, if there exists a necessity of applying a lead-free solder with high melting temperature.
The need of TIM applied for the second-stage thermal interface is mainly to maintain stable and high thermal conduction efficiency. Thus, the broader the range of melting temperature from solid to liquid of LMA, the better the interface performance of LMA. Therefore, a LMA with much broader range of melting temperature will be easier to keep in semisolid state to efficiently bring the desired latent heat benefits of fusion. The interface temperature outside the packaged semiconductors ranges mostly between 40° C. and 100° C. Due to ROHS regulated by the European Union, known LMAs which have melting point within said interfacial temperature range are only eutectic In—Bi—Sn alloy, eutectic In—Bi alloy and eutectic Bi—In—Sn alloy. In other words, the heat-conductive progress of above-mentioned alloys is still inherently constrained by narrow melting temperature range. When the interfacial temperature is higher than eutectic temperature of the eutectic alloy, the eutectic alloy will be easily heated into liquid state, resulting in melt overflow at the thermal interface. When the interfacial temperature is less than the eutectic temperature of the eutectic alloy, irregular surface voids of the thermal interface should be filled in advance by liquid phase of the eutectic alloy for subsequent interface application. A method called as “burn in” is often used so as to fill the interface surface voids as applying the eutectic alloy to a interface. However, the reflow heating is generally applied in chip package, so the “burn in” process in the second-stage thermal interface will increase the assembled complexity of packaged semiconductors. Furthermore, the eutectic alloys need to “burn in” hardly achieve the latent heat effect of melt.
To sum up, conventional LMAs are limited by eutectic point that is hard to adjust and narrow temperature range of fusion, so that only few alloys are accepted for thermal interface application of specific packaged semiconductors.
Accordingly, there is a need to develop a metal TIM having an adjustable melting temperature and/or a broad temperature range of melting, which is very much suitable for the thermal interface of the varying packaged semiconductors.